Lasers undergird a wide variety of products and applications in fields such as optoelectronics, medicine, environmental sensing, and manufacturing because of the unique properties of the coherent emission (light) they produce. Semiconductor lasers have proven to be of particular value because of their generally small size and cost, but for some applications the beam quality (beam divergence and mode) leaves much to be desired. Coupling semiconductor laser radiation into an optical fiber, for example, is often complicated by the aspect ratio of the output beam. The output of a semiconductor laser may diverge in an elliptical pattern, and collimation and beam shaping is generally required to couple the laser radiation into an optical fiber, for example, or to make use of it in other applications.
It has long been known that individual lasers can be phase-locked, yielding an array consisting of two or more individual sources that behaves as though it were a single emitter. Phased arrays are ubiquitous in science and engineering because a properly phased collection of sub-systems emulates a single large system. Ripper and Paoli demonstrated phase-locking of semiconductor lasers in 1970 (J. E. Ripper and T. L. Paoli, Appl. Phys. Lett. 17, 371 (1970)) and Schlossberg described in U.S. Pat. No. 4,367,554 the phase-locking of CO2 lasers. Today, arrays of CO2 lasers locked in phase are able to produce hundreds of watts of power in the infrared from an exceptionally small package.
It should be emphasized that phase-locking of these and other lasers is motivated by the fact that phase locking allows the output of a number of individual sources, each producing modest power, to be combined to yield much higher powers while still preserving the phase characteristics of a single high quality source. However, the prior art of phase-locked optical sources considers lasers exclusively. Optical sources are often characterized by the degree of spatial coherence they possess, which is generally expressed in terms of the coherence length. Most lasers have a high degree of spatial coherence and phase locking an ensemble of lasers usually requires locating the individual lasers within one coherence length of each other.